Using optical signals as a means of carrying channeled information at high speeds through an optical path such as an optical waveguide i.e. optical fibres, is preferable over other schemes such as those using microwave links, coaxial cables, and twisted copper wires, since in the former, propagation loss is lower, and optical systems are immune to Electro-Magnetic Interference (EMI), and have higher channel capacities. High-speed optical systems have signaling rates of several mega-bits per second to several tens of giga-bits per second.
Optical communication systems are nearly ubiquitous in communication networks. The expression herein "Optical communication system" relates to any system that uses optical signals at any wavelength to convey information between two points through any optical path. Optical communication systems are described for example, in Gower, Ed. Optical communication Systems, (Prentice Hall, N.Y.) 1993, and by P. E. Green, Jr in "Fiber optic networks" (Prentice Hall New Jersey) 1993, which are incorporated herein by reference.
As communication capacity is further increased to transmit an ever-increasing amount of information on optical fibres, data transmission rates increase and available bandwidth becomes a scarce resource.
High speed data signals are plural signals that are formed by the aggregation (or multiplexing) of several data streams to share a transmission medium for transmitting data to a distant location. Wavelength Division Multiplexing (WDM) is commonly used in optical communications systems as means to more efficiently use available resources. In WDM each high-speed data channel transmits its information at a pre-allocated wavelength on a single optical waveguide. At a receiver end, channels of different wavelengths are generally separated by narrow band filters and then detected or used for further processing. In practice, the number of channels that can be carried by a single optical waveguide in a WDM system is limited by crosstalk, narrow operating bandwidth of optical amplifiers and/or optical fiber non-linearities. Moreover such systems require an accurate band selection, stable tunable lasers or filters, and spectral purity that increase the cost of WDM systems and add to their complexity. This invention relates to a method and system for filtering or separating closely spaced channels that would otherwise not be suitably filtered by conventional optical filters.
Currently, internationally agreed upon channel spacing for high-speed optical transmission systems, is 100 Ghz, equivalent to 0.8 nm, surpassing, for example 200 Ghz channel spacing equivalent to 1.6 nanometers between adjacent channels. Of course, as the separation in wavelength between adjacent channels decreases, the requirement for more precise demultiplexing circuitry capable of ultra-narrow-band filtering, absent crosstalk, increases. The use of conventional dichroic filters to separate channels spaced by 0.4 nm or less without crosstalk, is not practicable; such filters being difficult if not impossible to manufacture.
In a paper entitled Multifunction optical filter with a Michelson-Gires-Turnois interferometer for wavelength-division-multiplexed network system applications, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device hereafter termed the GT device provides some of the functionality provided by the instant invention. For example, the GT device as exemplified in FIG. 1 serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror 16. The etalon 10 used has a 99.9% reflective back reflector 12r and a front reflector 12f having a reflectivity of about 10%; hence an output signal from only the front reflector 12f is utilized. A beam splitting prism (BSP) 18 is disposed to receive an incident beam and to direct the incident beam to the etalon 10. The BSP 18 further receives light returning from the etalon and provides a portion of that light to the plane mirror 16 and a remaining portion to an output port. Although this known GT device appears to perform its intended function, it appears to have certain limitations: As opposed to the device of the instant invention which has a phase difference between transmitted and reflected E-fields of 0 and .pi. radians, the GT as can be seen in the graph of FIG. 1b has some variation in phase between 0 and .pi. radians. Furthermore, in the GT device a finite optical path difference is required in the interferometer to produce an output response that mimics the one provided by the device of the instant invention. Typically for a 50 GHz free spectral range (FSR) this optical path difference would be a few millimeters; in contrast in the instant invention the optical phase difference need only be approximately .lambda./4 resulting in a more temperature stable and insensitive system. One further limitation of the GT device is its apparent requirement in the stabilization of both the etalon and the interferometer. Yet a further drawback to the GT device is the requirement for an optical circulator to extract the output signal adding to signals loss and increased cost of the device; and the requirement of a BSP which is known to have a significant polarization dependent loss. FIG. 1a shows a graph with a linear plot of the phase difference between the two reflected E-fields from the GT and a mirror with an optical path difference. Further, the graph shows a linear plot of phase variation of reflected light from the GT. A reflection profile is also shown in a logarithmic plot, and a straight sloped dotted line is a linear plot of a GT with a finite optical path difference.
It is an object of this invention to provide a method and circuit for separating an optical signal having closely spaced channels into at least two optical signals wherein channel spacing between adjacent channels is greater in each of the at least two optical signals, thereby requiring less precise filters to demultiplex channels carried by each of the at least two signals.
The present invention is believed to overcome many of the limitations of the prior art GT device and of other known multiplexing and demultiplexing devices.
It is an object of this invention to provide a relatively inexpensive optical circuit for interleaving or de-interleaving optical channels.
It is an object of this invention to provide an etalon based device wherein output signals from two oppositely disposed ports can be controllably interferometrically combined to yield a desired output response.